Chapter 10

This chapter was copied with permission from Nick Strobel’s Astronomy Notes. Go to his site at for the updated and corrected version.

Solar System Fluff

The basics of meteorites, asteroids, and comets are introduced and how they can tell us the “when” and the “how” of the formation of the solar system. At the end is an exploration of the other planetary systems. Updates on Comet Hartley discoveries, exoplanet discoveries including those from the Kepler mission and free floater exoplanets, and early history of the solar system.

10.1 Introduction

This chapter covers all of the solar system that is not the planets or the Sun: meteorites, asteroids, and comets. I group them all together as “solar system fluff” because the objects are much smaller than planets and most moons. The “fluff” may be small in size but certainly not in importance. We get clues of the origin of the solar system from these small objects. The chapter will end with the current model for the formation of the solar system and explorations of other planetary systems. The vocabulary terms are in boldface.

10.2 Asteroids

There are thousands, even millions, of small rocks that orbit the Sun, most of them between the orbits of Mars and Jupiter. Some asteroids called Trojan asteroids travel in or near Jupiter’s orbit about 60 degrees ahead of Jupiter and 60 degrees behind Jupiter (gravity balance points between Jupiter and the Sun). Some asteroids have orbits that bring them close to Earth’s orbit, some even crossing the Earth’s orbit. These are called Near-Earth Asteroids (NEAs) and include some 1132 (at time of writing) “Potentially Hazardous Asteroids” with the greatest potential of very close approaches to Earth. Comets that get near the Earth and NEAs are lumped together in Near-Earth Objects (NEOs). A plot of the known asteroids is available at the Minor Planet Center.

About one million of them are larger than 1 kilometer across. Those smaller than about 300 kilometers across have irregular shapes because their internal gravity is not strong enough to compress the rock into a spherical shape. The largest asteroid is Ceres with a diameter of 1000 kilometers. Pallas and Vesta have diameters of about 500 kilometers and about 15 others have diameters larger than 250 kilometers. The number of asteroids shoots up with decreasing size. The combined mass of all of the asteroids is about 32 times less than the Moon’s mass (with Ceres making up over a third of the total). Very likely the asteroids are pieces that would have formed a planet if Jupiter’s strong gravity had not stirred up the material between Mars and Jupiter. The rocky chunks collided at speeds too high to stick together and grow into a planet.

where the asteroids are

Though there are over a million asteroids, the volume of space they inhabit is very large, so they are far apart from one another. Unlike the movie The Empire Strikes Back and other space movies, where the spacecrafts flying through an asteroid belt could not avoid crashing into them, real asteroids are at least tens of thousands of kilometers apart from each other. Several spacecraft sent to the outer planets have traveled through the asteroid belt with no problems (and no swerving about).

There are three basic types of asteroids:

  1. C: they are carbonaceous—made of silicate materials with a lot of carbon compounds so they appear very dark. They reflect only 3 to 4% of the sunlight hitting them. You can tell what they are made of by analyzing the spectra of sunlight reflecting off of them. This reflectance spectra shows that they are primitive, unchanged since they first solidified about 4.6 billion years ago. A sizable fraction of the asteroids are of this type. The asteroid called Mathilde, explored by the NEAR spacecraft is an example of this type (see picture below).
  2. S: they are made of silicate materials without the dark carbon compounds so they appear brighter than the C types. They reflect about 15 to 20% of the sunlight hitting them. Most of them appear to be primitive and they make up a smaller fraction of the asteroids than the C types. Gaspra and Ida, explored by the Galileo spacecraft on its way to Jupiter, and Eros, orbited by the NEAR spacecraft for a year, are examples of this type (see picture below).
  3. M: they are made of metals like iron and nickel. These rare type of asteroids are brighter than the S and C types. We think they are the remains of the cores of differentiated objects. Large objects were hot enough in the early solar system so that they were liquid. This allowed the dense materials like iron and nickel to sink to the center while the lighter material like ordinary silicate rock floated up to the top. Smaller objects cooled off quicker than larger objects, so they underwent less differentiation. In the early solar system, collisions were much more common and some of the differentiated large asteroids collided with one another, breaking them apart and exposing their metallic cores.

asteroids visited by spacecraft
Two of the three types of asteroids are represented by the asteroids that have been explored up close with spacecraft. Mathilde is a dark C-type (brightness enhanced several times to match the other three). Gaspra, Ida, and Eros are S-type asteroids. Mathilde and Eros were visited by NEAR Shoemaker and Gaspra and Ida were visited by Galileo.

Note their irregular shapes! Small bodies can have irregular shapes because their gravity is too weak to crush the material into the most compact shape possible: a sphere. Depending on the strength of the material of which they are made, the largest non-spherical asteroids (and moons) can have diameters of roughly 360 to 600 kilometers. Planets are much too large (have too much gravity) to be anything but round. Ceres, the largest asteroid, is large enough to be round and is now re-classified as a “dwarf planet” (along with Pluto, Charon, and Eris).

The Japanese Aerospace Exploration Agency has a spacecraft called Hayabusa en route back to Earth after orbiting and landing on Itokawa, a small near-Earth asteroid only half a kilometer in length. Hayabusa collected at least one sample from the asteroid’s surface and will return to Earth in June 2010. Below are images of Itokawa from Hayabusa when it was just 7 kilometers from the asteroid. It has a rough surface but very few impact craters. Itokawa is basically a rubble pile formed by the ejecta from a large impact on a larger object coming back together gravitationally.

itokawa image from Hayabusa

Itokawa + 270 deg surface

other side of Itokawa

Itokawa + 90 deg surface

In late September 2007, NASA launched the DAWN spacecraft to explore the two largest asteroids, Ceres (about 960 km in diameter) and Vesta (520 km in diameter), for about six months at each asteroid. Vesta will be explored from August 2011 to May 2012 and Ceres will be explored from February 2015 to July 2015. Below are the best pictures we have of these asteroids (the Vesta image is a 3D computer model derived from Hubble Space Telescope data) and how they compare to the much smaller Eros asteroid that was explored by the NEAR Shoemaker spacecraft. Its primary goal is to help us figure out the role of size and water in determining the evolution of the planets. Ceres is a primitive and relatively wet protoplanet while Vesta has changed since its formed and is now very dry. At nearly the same distance from the Sun, why did these two bodies become very different?

Ceres and Vesta HST images with Eros for comparison

A few other asteroids have surfaces made of basalt from volcanic lava flows. When asteroids collide with one another, they can chip off pieces from each other. Some of those pieces, called meteoroids, can get close to the Earth and be pulled toward the Earth by its gravity.

10.3 Meteorites

The quick flashes of light in the sky most people call “shooting stars” are meteors—pieces of the rock glowing from friction with the atmosphere as they plunge toward the surface at speeds around 20 to 40 kilometers/second. Most of the meteors you see are about the size of a grain of sand and burn up at altitudes above 50 kilometers (in the mesosphere). More meteors are seen after midnight because your local part of the Earth is facing the direction of its orbital motion around the Sun. Meteoroids moving at any speed can hit the atmosphere. Before midnight your local part of the Earth is facing away from the direction of orbital motion, so only the fastest moving meteoroids can catch up to the Earth and hit the atmosphere. The same sort of effect explains why an automobile’s front windshield will get plastered with insects while the rear windshield stays clean.

Why more meteors seen after midnight

If the little piece of rock makes it to the surface without burning up, it is called a meteorite. There are three basic types of meteorites.

10.3.1 Stones

Stones: they are made of silicate material with a density around 3 (relative to the density of water) and look like ordinary Earth rocks. This makes them hard to distinguish from Earth rocks so they do not stand out. About 95% to 97% of the meteorites are of these type. About 85% of the stones are primitive, unchanged since they first solidified about 4.6 billion years ago. Most of the primitive stones have chondrules—round glassy structures 0.5 to 5 millimeters across embedded in the meteorites. They are solidified droplets of matter from the early solar nebula and are the oldest part of a primitive meteorite.

chondrite meteorite
A meteorite containing chondrules (courtesy of the Planetary Data Center). The oldest of the stone meteorites are the carbonaceous meteorites. They contain silicates, carbon compounds (giving them their dark color), and a surprisingly large amount of water (about 22% of their mass). They are probably chips of C-type asteroids. Some of the carbonaceous meteorites have organic molecules called amino acids. Amino acids can be connected together to form proteins that are used in the biological processes of life. There is the possibility that meteorites like these may have been the seeds of life on the Earth. In addition, these type of meteorites may have provided the inner planets with a lot of water. The terrestrial planets may have been so hot when they formed that most of the water in them at formation evaporated away to space. The impact of millions to billions of carbonaceous meteorites (and comets) in the early solar system may have replenished the water supply on the terrestrial planets.

About 10% to 12% of the stones are from the crust of differentiated parent objects. Therefore, they are younger (only 4.4 billion years old). The lighter-colored stones are chips from the S-type of asteroids.

10.3.2 Stoney-Irons

Stoney-Irons: only 1% of the meteorites are of this type. They have a variable mixture of metal (iron and nickel) and rock (silicates) and have densities ranging from 4 to 6 times that of water. They come from a differentiated object at the boundary between the metal core and the rock crust. They are 4.4 billion years old.

10.3.3 Irons

Irons: although they make up about 40% of the meteorites found worldwide, only 2 to 3% of the meteorites are these type. They make up so many of the ones found because they are easily distinguished from Earth rocks. They are noticeably denser than Earth rocks, they have a density around 7 times that of water. They come from the core of a differentiated body and are made of iron and nickel. They are 4.4 billion years old. Irons sometimes have large, coarse-grained crystalline patterns (“Widmanstatten patterns”) that is evidence that they cooled slowly.

iron meteorite

An iron meteorite (courtesy of the Planetary Data Center).

The primitive meteorites are probably the most important ones because they hold clues to the composition and temperature in various parts of the early solar nebula. Because of this, some astronomers put the meteorites into two groups: primitive and processed (not primitive).

10.3.4 Finding Meteorites

Most stoneys look like Earth rocks and so they are hard to spot. The rare irons are easy to distinguish from Earth rocks and make up most of the ones found worldwide. Usually the meteorites that science museums show off are iron meteorites. Not only does their high density and metal composition set them apart from ordinary rocks, the iron meteorites are stronger. This means they will more likely survive the passage through the atmosphere in one piece to make impressive museum displays. The stone meteorites are more fragile and will break up into several pieces (less impressive for museum displays).

better contrast in Antarctica

To get an accurate number for the proportion of meteorites that fall to the Earth (an unbiased sample), meteorite searchers go to a place where all types of rocks will stand out. The best place to go is Antarctica where the stable, white ice pack makes darker meteorites easy to find. Meteorites that fell thousands of years ago can still be found in Antarctica without significant weathering. Since the 1980s, thousands of meteorites have come from here. For further exploration, check out the Antarctica Meteorite web site at the Planetary Materials Curation office of NASA and the ANSMET site at Case Western Reserve University.

finding meteorites in Antarctica
Meteorites stand out against the snow and ice background of Antarctica.

Most meteorites are pieces of asteroids, but 108 (at the time of writing) are from the Moon. A select few (45 at the time of writing), the Shergotty-Nakhla-Chassigny (SNC) meteorites, may be from Mars. The relative abundances of magnesium and heavy nitrogen (N-15) gases trapped inside the SNC meteorites is similar to the martian atmosphere as measured by the Viking landers and unlike any meteorites from the asteroids or Moon. Also, the isotope ratios of argon and xenon gas trapped in the meteorites most closely resemble the martian atmosphere and are different than the typical meteorite. The analysis of the soil and rocks by the Mars Pathfinder confirm this.

SNC meteorite (EETA 79001)
A meteorite blasted from Mars.

Most SNC meteorites are about 1.4 Gyr old, but the one with suggestions of extinct Martian life is about 4.5 Gyr old. The discovery was published in the August 16, 1996 issue of the journal Science. Non-subscribers can find a copy of the article here. Recent studies of the meteorite have cast considerable doubt on the initial claims for fossil microbes in the rock. There is strong evidence of contamination by organic molecules from Earth, so this meteorite does not provide the conclusive proof hoped for. A detailed description of SNC meteorites is given at JPL’s Mars Meteorites web site.

10.4 Radioactive Dating

There are several ways to figure out relative ages, that is, if one thing is older than another. For example, looking at a series of layers in the side of a cliff, the younger layers will be on top of the older layers. Or you can tell that certain parts of the Moon’s surface are older than other parts by counting the number of craters per unit area. The old surface will have many craters per area because it has been exposed to space for a long time. But how old is “old”? If you assume that the impact rate has been constant for the past several billion years, then the number of craters will be proportional to how long the surface is exposed. However, the crater number relation must be calibrated against something with a known age.

To measure the passage of long periods of time, scientists take advantage of a regularity in certain unstable atoms. In radioactive atoms the nucleus will spontaneously change into another type of nucleus. When looking at a large number of atoms, you see that a certain fraction of them will change or decay in a certain amount of time that depends on the type of atom—more specifically, the type of nucleus. Radioactive dating is an absolute dating system because you can determine accurate ages from the number of remaining radioactive atoms in a rock sample. Most of the radioactive isotopes used for radioactive dating of rock samples have too many neutrons in the nucleus to be stable.

An isotope is a particular form of an element. All atoms of an element have the same number of protons in their nucleus and behave the same way in chemical reactions. The atoms of an isotope of a given element have the same number of protons AND neutrons in their nucleus. Different isotopes of a given element will have the same chemistry but behave differently in nuclear reactions. In a radioactive decay, the original radioactive isotope is called a parent isotope and the resulting isotope after the decay is called a daughter isotope. For example, Uranium-238 is the parent isotope that breaks apart to form the daughter isotope Lead-204.

10.4.1 Radioactive Dating Method

standard exponential decay curve decrease by one-half every half-life

Radioactive isotopes will decay in a regular exponential way such that one-half of a given amount of parent material will decay to form daughter material in a time period called a half-life. A half-life is NOT one-half the age of the rock! When the material is liquid or gaseous, the parent and daughter isotopes can escape, but when the material solidifies, they cannot so the ratio of parent to daughter isotopes is frozen in. The parent isotope can only decay, increasing the amount of daughter isotopes. Radioactive dating gives the solidification age.

There are two simple steps for radioactive dating:

  1. Find out how many times you need to multiply (1/2) by itself to get the observed fraction of remaining parent material. Let the number of the times be n. For example 1/8 = (1/2) × (1/2) × (1/2), so n = 3. The number n is the number of half-lives the sample has been decaying. If some material has been decaying long enough so that only 1/4 of the radioactive material is left, the sample is 2 half-lives old: 1/4 = (1/2) × (1/2), n =2.
  2. The age of the sample in years = n × (one half-life in years).

How do you do that?

If 1/8 of the original amount of parent isotope is left in a radioactive sample, how old is the sample? Answer: After 1 half-life, there is 1/2 of the original amount of the parent left. After another half-life, there is 1/2 of that 1/2 left = 1/2 × 1/2 = 1/4 of original amount of the parent left. After yet another half-life, there is 1/2 of that 1/4 left = 1/2 × 1/2 × 1/2 = 1/8 of the original amount of the parent left (which is the fraction asked for). So the rock is 1 half-life + 1 half-life + 1 half-life = 3 half-lives old (to get the age in years, simply multiply 3 by the half-life in years).

If you have a fraction that is not a multiple of 1/2, then it is more complicated. The age = [ln(original amount of parent material / current amount of parent material) / ln(2)] × (half-life in years), where ln() is the “natural logarithm” (it is the “ln” key on a scientific calculator).

10.4.2 If Amount of Original Is Not Known

There are always a few astronomy students who ask me the good question (and many others who are too shy to ask), “what if you don’t know the original amount of parent material?” or “what if the rock had some daughter material at the very beginning?” The age can still be determined but you have to be more clever in determining it.

One common sense rule to remember is that the number of parent isotope atoms + the number of daughter isotope atoms = an unchanging number throughout time. The number of parent isotopes decreases while the number of daughter isotopes increases but the total of the two added together is a constant. You need to find how much of the daughter isotopes in the rock (call that isotope “A” for below) are not the result of a radioactive decay of parent atoms. You then subtract this amount from the total amount of daughter atoms in the rock to get the number of decays that have occurred since the rock solidified. Here are the steps:

  1. Find another isotope of the same element as the daughter that is never a result of radioactive decay (call that isotope “B” for below). Isotopes of a given element have the same chemical properties, so a radioactive rock will incorporate the NON-radioactively derived proportions of the two isotopes in the same proportion as any nonradioactive rock.
  2. Measure the ratio of isotopes A and B in a nonradioactive rock. This ratio, R, will be the primitive (initial) proportion of the two isotopes.
  3. Multiply the amount of the non-daughter isotope (isotope B) in the radioactive rock by the ratio of the previous step: (isotope B) × R = initial amount of daughter isotope A that was not the result of decay.
  4. Subtract the initial amount of daughter isotope A from the rock sample to get the amount of daughter isotope A that IS due to radioactive decay. That number is also the amount of parent that has decayed (remember the rule #parent + #daughter = constant). Now you can determine the age as you did before.

The oldest meteorites have ages clustering around 4.55 billion years with uncertainties in the age measurements of less than 100 million years.

The discussion above is for the case of determining when a rock solidified (and it is usually very old rocks!). To determine the ages of old, once-living material such as plants, then something like carbon-14 will be used. Most carbon atoms are carbon-12 (99%) or carbon-13 (1%). A very small fraction (about 1 part in 1012 ) are the radioactive carbon-14 isotope that will decay to form nitrogen-14 with a half-life of 5,730 years. Carbon-14 is being produced continuously in our atmosphere when cosmic rays (extremely high-energy particles from space, mostly protons) collide with air molecules. When plants absorb carbon-dioxide in the photosynthesis process, some of the carbon dioxide has the carbon-14 atom in the molecule. Assuming that our atmosphere’s composition and the cosmic ray flux has not changed significantly in the last few thousand years, you can find the age of the once-living organic material by comparing its carbon-14/carbon-12 ratios to those of now-living plants. Carbon-14 dating works well for samples less than about 50,000 to 60,000 years old and for things that were getting their carbon from the air.

isotopes of carbon

10.4.3 Is Radioactive Dating Valid?

The long ages (billions of years) given by radioactive dating of rocks seems an impossibly long time for some people. Since radioactive rocks have been observed for only a few decades, how do you know you can trust these long half-lifes and the long ages derived? Here are some points to consider:

  1. The rate of decay should follow a simple exponential decline based on the simple theory of probability in statistics. This same probability theory is used to figure the odds of winning by gamblers.
  2. An exponential decay is seen for short-lived isotopes with half-lives of only a few days.
  3. For the decades they have been observed, the long-lived isotopes also follow an exponential decay.
  4. The gamma ray frequencies and intensities produced by radioactive elements in supernova remnants change in the same predictable way as they do here on the Earth. One well-studied supernova remnant is SN1987A that is 169,000 light years away in a satellite galaxy of the Milky Way. The predictions for the decay rates have turned out to be correct for all of the radioactive elements we have detected in that remnant. Since the SN1987A is 169,000 light years away, that tells us the decay rates were not different 169,000 years ago. We find similar results for supernova remnants even further away (and therefore, further back in time).
  5. The decay probability should not depend on time because:
    • An exponential decay IS observed for short-lived isotopes.
    • Decays are nuclear reactions. Nuclear reactions only care about size scales of 10-13 centimeters (100 million times smaller than the wavelength of visible light). The composition and state of the surrounding material will not affect the rate of decay.
    • Tests looking for a variable decay probability by changing the pressure, temperature, and chemical combinations of the surrounding material have not found any variation in the decay probability. The decay rates do not change under all of the conditions tested.
    • The laws of nature or physics at the nuclear level should not change with time.
    • Astronomical observations show that the laws of physics are the same everywhere in the universe and have been unchanged for the past 13.7 billion years.

The American Astronomical Society and the Astronomical Society of the Pacific published a beautifully-illustrated guide for teachers, students, and the public called An Ancient Universe: How Astronomers Know the Vast Scale of Cosmic Time. (PDF document: 800 kB in size!) This guide for Teachers, Students and the Public was written by a subcommittee of the American Astronomical Society’s Astronomy Education Board. This is a local copy from the AAS Education Board.

10.5 Effects of an Asteroid Impact on Earth

Some asteroids have orbits that cross the orbit of the Earth. That means that the Earth will be hit sometime. Recent studies have shown that the Earth has been hit an alarmingly large number of times in the past. One large impact is now thought to have contributed to the quick demise of the dinosaurs about 65 million years ago. What would be the effects of an asteroid hitting the Earth?

the battered Earth

Known impact sites on the Earth’s continents (from The Planetary and Space Science Centre). See also LPI’s Terrestrial Impact Site for pictures of the craters.

What follows is a condensation of an excellent article by Sydney van den Bergh called “Life and Death in the Inner Solar System” in the May 1989 issue of the Publications of the Astronomical Society of the Pacific (vol. 101, pages 500-509). He considers a typical impact scenario of a 10-kilometer object with density = 2.5 times that of water, impacting at a speed of 20 kilometers/second. Its mass = 1.31 trillion tons (1.31 × 1015 kilograms). A 1-kilometer object has a mass = 1.31 billion tons.

10.5.1 Explosion

Obviously, something this big hitting the Earth is going to hit with a lot of energy! Let’s use the energy unit of 1 megaton of TNT (=4.2× 1015 Joules) to describe the energy of the impact. This is the energy one million tons of dynamite would release if it was exploded and is the energy unit used for nuclear explosions. The largest yield of a thermonuclear warhead is around 50–100 megatons. The kinetic energy of the falling object is converted to the explosion when it hits. The 10-kilometer object produces an explosion of 6 × 107 megatons of TNT (equivalent to an earthquake of magnitude 12.4 on the Richter scale). The 1-kilometer object produces a milder explosion of “only” 6 × 104 megatons (equivalent to an earthquake of magnitude 9.4 on the Richter scale).

On its way to the impact, the asteroid pushes aside the air in front of it creating a hole in the atmosphere. The atmosphere above the impact site is removed for several tens of seconds. Before the surrounding air can rush back in to fill the gap, material from the impact — vaporized asteroid, crustal material, and ocean water (if it lands in the ocean) — escapes through the hole and follows a ballistic flight back down. Within two minutes after impact, about 105 cubic kilometers of ejecta (1013 tons) is lofted to about 100 kilometers. If the asteroid hits the ocean, the surrounding water returning over the the hot crater floor is vaporized (a large enough impact will break through to the hot lithosphere and maybe the even hotter asthenosphere), sending more water vapor into the air as well as causing huge steam explosions that greatly compound the effect of the initial impact explosion.

There will be a crater regardless of where it lands. The diameter of the crater in kilometers is = (energy of impact)(1/3.4)/106.77. Plugging in the typical impact values, you get a 150-kilometer diameter crater for the 10-kilometer asteroid and a 20-kilometer diameter crater for the 1-kilometer asteroid. The initial blast would also produce shifting of the crust along fault lines.

Meteor (Barringer) Crater in northern Arizona (about 1 kilometer across). Select here for a view from the rim.

Chicxulub Crater in Yucatan, Mexico (from the one that may have killed off the dinosaurs).

10.5.2 Tsunami

The oceans cover about 75% of the Earth’s surface, so it is likely the asteroid will hit an ocean. The amount of water in the ocean is nowhere near large enough to “cushion” the asteroid. The asteroid will push the water aside and hit the ocean floor to create a large crater. The water pushed aside will form a huge tidal wave, a tsunami. The tidal wave height in meters = (distance from impact)-0.717 × (energy of impact)0.495/ (1010.17). What this means is that a 10-km asteroid hitting any deep point in the Pacific (the largest ocean) produces a megatsunami along the entire Pacific Rim.

Some values for the height of the tsunami at different distances from the impact site are given in the following table. The heights are given for the two typical asteroids, a 10-kilometer and a 1-kilometer asteroid.

Distance (in km) 10 km 1 km
300 1.3 km 43 m
1000 540 m 18 m
3000 250 m 3 m
10000 100 m 3 m

The steam blasts from the water at the crater site rushing back over the hot crater floor will also produce tsunamis following the initial impact tsunami and crustal shifting as a result of the initial impact would produce other tsunamis—a complex train of tsunamis would be created from the initial impact (something not usually shown in disaster movies).

large asteroid hits the Earth 65 million years ago

10.5.3 Global Firestorm

The material ejected from the impact through the hole in the atmosphere will re-enter all over the globe and heat up from the friction with the atmosphere. The chunks of material will be hot enough to produce a lot of infrared light. The heat from the glowing material will start fires around the globe. Global fires will put about 7 × 1010 tons of soot into the air. This would “aggravate environmental stresses associated with the … impact.”

10.5.4 Acid Rain

The heat from the shock wave of the entering asteroid and reprocessing of the air close to the impact produces nitric and nitrous acids over the next few months to one year. The chemical reaction chain is:

  1. N2 + O2 ‚> NO (molecular nitrogen combined with molecular oxygen produces nitrogen monoxide)
  2. 2NO + O2 ‚> 2NO2 (two nitrogen monoxide molecules combined with one oxygen molecule produces two nitrogen dioxide molecules)
  3. NO2 is converted to nitric and nitrous acids when it is mixed with water.

These are really nasty acids. They will wash out of the air when it rains—a worldwide deluge of acid rain with damaging effects:

  • destruction or damage of foliage;
  • great amounts of weathering of continental rocks;
  • the upper ocean organisms are killed. These organisms are responsible for locking up carbon dioxide in their shells (calcium carbonate) that would eventually become limestone. However, the shells will dissolve in the acid water. That along with the “impact winter” (described below) kills off about 90% of all marine nanoplankton species. A majority of the free oxygen from photosynthesis on the Earth is made by nanoplankton.
  • The ozone layer is destroyed by O3 reacting with NO. The amount of ultraviolet light hitting the surface increases, killing small organisms and plants (key parts of the food chain). The NO2 causes respiratory damage in larger animals. Harmful elements like Beryllium, Mercury, Thallium, etc. are let loose.

10.5.5 Temperature Effects

All of the dust in the air from the impact and soot from the fires will block the Sun. For several months you cannot see your hand in front of your face! The dramatic decrease of sunlight reaching the surface produces a drastic short-term global reduction in temperature, called impact winter. Plant photosynthesis stops and the food chain collapses.

The cooling is followed by a much more prolonged period of increased temperature due to a large increase in the greenhouse effect. The greenhouse effect is increased because of the increase of the carbon dioxide and water vapor in the air. The carbon dioxide level rises because the plants are burned and most of the plankton are wiped out. Also, water vapor in the air from the impact stays aloft for awhile. The temperatures are too warm for comfort for awhile.

In the early 1990s astronomers requested funding for an observing program called Spaceguard to catalog all of the near-Earth asteroids and short period comets. The international program would take 10 years to create a comprehensive catalog of all of the hazardous asteroids and comets. The cost for the entire program (building six special purpose telescopes and operation costs for ten years) would be less than what it costs to make a popular movie like Deep Impact or Armageddon.

In mid-1999 NASA and the US Air Force began a Near-Earth Object search program using existing telescopes to locate 90% of the NEOs larger than 1 kilometer in diameter in ten years. As of October 25, 2011, the program has found 831 asteroids larger than 1 kilometer in diameter and there are 1260 “Potentially Hazardous Asteroids” with diameters greater than 150 meters (see the NEO Discovery Statistics page for updates). To find out more about the United States’ program go to NASA’s Asteroid and Comet Hazards site and JPL’s Asteroid Watch (site intended for the general public with educational materials to learn about asteroids) or Near-Earth Object Program (site with more in-depth information but not as glitzy as the Asteroid Watch site). One process that affects the orbits of asteroids and, therefore, introduces uncertainty in whether a particular NEA will hit the Earth is the Yarkovsky effect. The afternoon emission of infrared energy from solar heating is not pointed right at the Sun, so the thermal radiation from the asteroid is not exactly balanced by the solar photons. This results in a pushing that can move the asteroid inward toward the Sun or away from the Sun. You can try out your hand at making big craters at the Solar System Collisions website and the Earth Impact Effects Program website (but, please, try not to wipe out the entire Earth).

Sections Review


  • carbonaceous meteorite
  • chondrules
  • differentiated
  • half-life
  • isotope
  • meteorite
  • primitive
  • radioactive dating

Review Questions 1

  1. Where are most of the asteroids found?
  2. Why are spacecraft able to pass through the asteroid belt without getting hit?
  3. What are the three types of asteroids and what are they made of?
  4. Why are more meteors seen after midnight?
  5. What are the proportions of the three types of meteorites and what are they made of?
  6. What type of asteroid does each type of meteorite correspond to?
  7. Which meteorites are primitive and why are they particularly important for understanding the origin of the planets?
  8. What makes carbonaceous meteorites so special?
  9. Why are chondrules especially important for solar system formation models? What type of meteorite are they likely to be found in?
  10. How old are the various types of meteorites and why are they used to find the age of the solar system?
  11. Why do iron meteorites make up 40% of the “finds,” but are only 2 to 3% of the total meteorites?
  12. Why is Antarctica a good place to get an unbiased sample of meteorites?
  13. What makes SNC meteorites so unique and how are they different than other types of meteorites?
  14. Does radioactive dating give us relative or absolute ages for rocks? What type of “age” does it tell us?
  15. How do you use the half-life to find the age of radioactive rocks?
  16. If the half-life of a radioactive sample is 1 month, is a sample of it completely decayed after 2 months? If not, how much is left?
  17. Uranium-235 has a half-life of 700 million years. How long will you have to wait until a 1-kilogram chunk decays so that only 0.0625 kilograms (1/16 kg) is left?
  18. How are the very old ages derived for some radioactive rocks known to be correct?
  19. If a large asteroid were to hit Earth, how much of the Earth’s surface would be affected?

10.6 Comets

The passage of the comet Hale-Bopp through our part of the solar system created spectacular displays in the spring of 1997. Every newspaper, television and radio station carried some report with photos of the comet. We had a foretaste of the Hale-Bopp show when Comet Hyakutake passed close to the Earth in the spring of 1996. Hale-Bopp was one of the brightest comets to grace our skies this century, coming close to the displays put on by Comet West in 1976 and Halley’s Comet in 1910. Many people were justifiably interested in Hale-Bopp—it was a gorgeous sight! Many people also tuned into the news and astronomy web sites in the summer of 1994 when the comet Shoemaker-Levy 9 smashed into the planet Jupiter. Predictions of Jupiter’s demise were, of course, greatly exaggerated—Jupiter took the hits in stride.

One view of comets as destroyers of worlds in 1857. (Original source unknown)

King Harold and nation cower in fear at the close passage of Halley’s Comet (picture from part of the Bayeux Tapestry; more pictures at The Bayeux Tapestry Gallery).

Our favorable view of comets is a big change from the dread and fear people held of comets even less than a century ago. Comets were usually thought to be omens of bad events to occur on the Earth. King Harold of England took the passage of Halley’s Comet to be a sign of his defeat in 1066. However, William the Conqueror took it as a good sign and led the Normans to victory over King Harold’s army. As recently as 1910 people thought the end of the world was near when it was discovered the Earth would pass through the tail of Halley’s Comet. Astronomers had discovered the presence of cyanogen molecules in the tail, so the popular media spread tales of cyanide poisoning of the Earth. Even with great effort the astronomers were not able to convince many people that we faced no danger—a comet’s tail is extremely diffuse so the minute amounts making it through the atmosphere and being breathed by helpless human beings was much, much less than the noxious stuff they breathed everyday from industrial pollution. The tragedy of the Heaven’s Gate cult shows that despite our current knowledge of comets, there are still those who view comets with great superstition or as something much more than the icy bodies they are from the outer limits of the solar system.

Comets are small “potato-shaped” objects a few hundred meters to about 20 kilometers across. They are made of dust grains embedded in frozen volatiles like water, methane, ammonia, and carbon dioxide (they are like “dirty icebergs”). They are primitive objects which means they are unchanged since they first solidified from the solar nebula about 4.6 billion years ago. Comets are frozen relics of the early solar system holding valuable information about the formation of the planets.

the parts of a comet when it nears the Sun

When a comet gets close enough to the Sun, it changes into something more spectacular. The picture above shows the parts of a comet that form when the cold “dirty iceberg” is warmed up by the Sun. This picture is courtesy of David Doody at JPL and is part of the Basics of Space Flight manual for all operations personnel.

10.6.1 Nucleus

All the material comes from the nucleus. This is the “dirty iceberg.” Comet nuclei are 0.5 to 20 kilometers in size and are potato-shaped conglomerate of dust (silicates and carbonaceous) embedded in ice (frozen water, carbon dioxide, carbon monoxide, methane, and ammonia). They have a mass of only 1014 to 1015 kilograms (the Earth has a mass of almost 6 × 1024 kilograms—tens to hundreds of billions of times larger than a comet). It is less than the size of a period on the scale of the comet drawing above. Because of their small size, comet nuclei have too little gravity to crush the material into a sphere.

When a comet nears the Sun around the Jupiter-Saturn distance, it warms up. The ices sublime—they change from solid to gas without going through a liquid phase (like the white mist coming from a block of frozen carbon dioxide, “dry ice”). Jets of material will shoot out from the nucleus. These jets can alter the comet’s orbit (remember Newton’s third law of motion?)

Since the orbit of Halley’s Comet is known so well, spacecraft were sent to it when it passed through our part of the solar system in 1986. Here is a close-up of Halley’s Comet. The spacecraft Giotto launched by the European Space Agency on July 2, 1985 reached Halley’s Comet on March 13, 1986 and snapped this photo from 25,660 kilometers (15,950 miles) away. It got to within 596 kilometers (370 miles) of the nucleus, passing by at 68 kilometers/second. The nucleus of Halley’s Comet has dimensions of 8×8×16 kilometers. The nucleus has a density of only 0.1 to 0.25 times the density of water and is very dark—it reflects only 4% of the sunlight (coal reflects about 6%). The density is about that of a loosely-compacted snowball and it is quite fragile—you could break a piece of the nucleus in two with your bare hands!

Nucleus of Comet Halley

Selecting the picture will take you to the Max Planck Institut für Aeronomie from where this image Halley’s nucleus came (will display in another window). The bright white jets on the left side of the nucleus are pointed in the direction of the Sun. Comet Hale-Bopp’s nucleus is larger than Halley’s nucleus—10 to 40 kilometers in size (about twice the size of Halley’s Comet’s nucleus) and is dust-rich. It began ejecting material when still at the distance of the outer planets, so it was discovered while still a couple of years from its perihelion passage in March of 1997. Comet Hyakutake (bright comet of spring 1996 that passed within 0.1 A.U. of the Earth) has a nucleus 1 to 3 kilometers in size. Four other comet nuclei that have been imaged up close by spacecraft are shown below: Comet Borrelly (about 8 km long) on the top left, Comet Wild (about 5 km in diameter) on the top right, Comet Tempel 1 (about 7.6 km x 4.9 km) before (bottom left) and after (bottom right) it got hit by the Deep Impact probe in July 2005 and Comet Hartley 2 (about 2.0 km x 0.8 km). Like Halley they too are very dark.

The Deep Impact mission was designed to investigate the interior of a comet by crashing a 370-kg copper impactor into the nucleus of Tempel 1 and analyzing the material shot out from the impact (copper is an element not expected to be in comets). Tempel 1 is made of a mixture of materials with high and low melting temperatures which tells us that the early solar nebula was more complex than previously thought. It has silicates like beach sand with olivine and peroxine that would have come from the inner solar system, carbonates and sulfide that would have come from part of the solar system further out, and ice (mostly water ice and a little carbon dioxide) from the outer solar system. Tempel 1 has a powdery surface layer a few tens of meters thick with most of the ices missing in that surface layer (“a marshmallow dipped in powered sugar”). There are a few smooth patches of water ice on the surface. Young surfaces right next to old, battered areas with a layered (slabs of ice) interior filled with pockets of empty space tell us the geological history of comets is more complex (and interesting!) than we initially thought.

The Deep Impact spacecraft has been renamed the EPOXI mission. It flew by comet Hartley 2 on November 4, 2010. Hartley 2 is about a quarter the size of Tempel 1. Its two ends are rough and knobbly from which spew jets of carbon dioxide (“dry ice”) and a smooth region between the two ends from which frozen water sublimates through the dust. The images were clear enough that astronomers could see jets spewing out from specific surface features and even the “snow” particles in the jets the size of golf balls to basketballs. This is the first comet for which frozen carbon dioxide has provided the jets—usually the jets are made from the sublimation of frozen water. Select the images to go to the websites from where these images came (both images courtesy of NASA/JPL/UMD). The last image in the set compares the sizes of all five of the comet nuclei that have been visited by spacecraft (as of 2010).

nucleus of Comet Borrelly
nucleus of Comet Wild
Comet Tempel 1 5 minutes before impact
Comet Tempel 1 67 seconds after impact

Comets that have been visited by spacecraft (as of 2010)

10.6.2 Coma

Gas and dust pouring out from the nucleus forms a huge atmosphere around the nucleus. This is the bright core, called a coma, you can see when you observe a comet from the Earth. It is 100,000s of kilometers across. Because the nucleus has such low gravity (you could jump off it!), it cannot hang onto the escaping dust and gas. NASA’s StarDust mission captured material from Comet Wild’s coma in early January 2004 and returned the microscopic dust grains embedded in aerogel to Earth in mid-January 2006. It found that Comet Wild like Comet Tempel is also made of a mixture of materials with high and low melting temperatures. This may be the result of material originally forming near the Sun that got ejected to the outer parts of the solar system nebula via the bipolar jets we see in many young, forming stars. Further analysis of the Comet Wild material has uncovered Glycine, an amino acid.

10.6.3 Tails

When the comet gets to around Mars’ distance from the Sun, the Sun’s radiation pushes the coma gas and dust away from the Sun to form the well-known tails of a comet. Usually, two tails will form, a bluish, straight ion tail and a more curved, yellow-white dust tail. A nice example of this are shown in the picture of Comet West below:

the two tails of Comet West
Courtesy of John Laborde

The Sun is constantly spewing out charged particles, called the solar wind, into the solar system. The solar wind travels along solar magnetic field lines extending radially outward from the Sun. Ultraviolet light from the Sun ionizes some of the gases in the coma. These charged particles (ions) are forced along magnetic field lines to form the ion tail millions of kilometers long. The blue ion tail acts like a “solar” wind sock. The ion tail always points directly away from the Sun, so when the comet is moving away from the Sun, its ion tail will be almost in front of it! The blue color is mostly from the light emitted by carbon monoxide ions but other types of ions also contribute to the light. Since the gas is so diffuse, the observed spectrum is an emission-line spectrum.

The dust tails forms from the solar photons colliding with the dust in the coma. The dust forms a long, curved tail that lies slightly farther out from the Sun than the nucleus’ orbit. The dust tail has a yellow-white color from reflected sunlight. Both of the tails will stretch for millions of kilometers. Because of the large amount of dust, Hale-Bopp’s tail was much brighter and whitish-yellow from reflected sunlight. Hyakutake’s tail was dimmer and blue-green in appearance because of the low amount of dust and proportionally more ions.

Comet Hale-Bopp was a spectacular comet in the spring 1997 sky. Courtesy of Darren Bly

10.6.4 Hydrogen Cloud

Some of the water vapor ejected in the jets from the nucleus is dissociated by solar ultraviolet light into oxygen and hydrogen. The hydrogen forms a huge cloud around the comet that can be tens of millions of kilometers across. If you include the hydrogen cloud and tails in describing the size of comets, they can be the largest things in the solar system. However, all of this is coming from a dirty snowball the size of a city.

10.7 Comet Orbits—Oort Cloud and Kuiper Belt

Comets can be divided into two basic groups depending on their orbital periods. There are long period comets with orbital periods that can be thousands to millions of years long, and short period comets with orbital periods less than about 200 years. Their alignments with the plane of the planet orbits is also different. The long period comet orbits are oriented in all different random angles while the short period comet orbits are within about 30 degrees of the solar system plane. These orbital characteristics point to two regions beyond the realm of the planets: the Oort Cloud and the Kuiper Belt.

where comets come from

10.7.1 Oort Cloud

The Oort Cloud is a large spherical cloud with a radius from 50,000 to 100,000 A.U. surrounding the Sun filled with billions to trillions of comets. It has not been directly observed. Its existence has been inferred from observations of long period comets. Long period comets have very elliptical orbits and come into the inner solar system from all different random angles (not just along ecliptic). Kepler’s third law says that they have orbital periods of 100,000s to millions of years. However, their orbits are so elliptical that they spend only 2 to 4 years in the inner part of the solar system where the planets are and most of their time at 50,000 to 100,000 A.U. With such long orbital periods their presence in the inner solar system is, for all practical purposes, a one-time event. Yet we discover several long period comets every year. This implies the existence of a large reservoir of comets. This was first noted by the Dutch astronomer Jan Oort in 1950 so the spherical comet reservoir was named after him. If Halley’s Comet’s mass is typical for comets, then the Oort Cloud could have a total mass greater than all of the planets added together (but less than the Sun).

At the great distances of the Oort Cloud, comets can be affected by the gentle gravitational tugs of nearby passing stars. The passing stars tug on the comets, “perturbing” their orbits, sending some of them into the inner solar system. The comets passing close to a Jovian planet are deflected by the planet’s gravity into an orbit with a shorter period, only decades long. Jupiter and Saturn tend to deflect long period comets completely out of the solar system (or gobble them up as Jupiter did with Shoemaker Levy-9). Uranus and Neptune tend to deflect the long period comets into orbits that stay within the solar system. Halley’s Comet may be an example of a deflected comet. Unlike other short period comets, Halley’s Comet’s orbit is retrograde.

The Oort cloud comets probably formed at the about the same distance as Uranus and Neptune from the Sun 4.6 billion years ago and were then deflected outward when they passed too close to the two planets. Comets forming at the distance of Jupiter and Saturn were either ejected from the solar system by these massive planets in a “gravitationally slingshot” or gobbled up. Comets forming further out than Neptune never coalesced to form a planet and now make up the Kuiper Belt.

10.7.2 Kuiper Belt

Using the observed characteristics of the short period comet orbits, the Dutch-American astronomer Gerard Kuiper proposed the existence of a disk of 100s of millions of comets from 30 to 100 or more A.U. from the Sun orbiting roughly along the ecliptic. This belt of comets, called the Kuiper Belt, was first observed in 1992. Comets originally from the Kuiper Belt that pass near the Earth have perihelia around the terrestrial planets’ distances from the Sun and aphelia beyond Neptune. Interactions with Neptune and Uranus have made their orbits so elliptical. Some examples are Comet Encke, Comet Giacobini-Zinner, and the former Comet Shoemaker-Levy 9.

The comets observed in the Kuiper Belt have more circular orbits and do not stray close to Uranus or Neptune. Many of the Kuiper belt comets observed from the ground are 100 to 300 kilometers in size (but some are Pluto-size) and orbit between 30 and 60 A.U. from the Sun. Another group of objects mostly between Saturn (9.5 A.U.) and Uranus (19.2 A.U.), called “Centaurs,” may be an extension of the Kuiper Belt. These objects include Chiron (170 km in diameter) and Chariklo (about 240 km in diameter) and many others.

Pluto and Charon as seen from HST

Because of its small size and low density, some astronomers view the planet Pluto (2330 kilometers in diameter and just 1/6th our Moon’s mass; on the left in the image above) as just a large comet. Pluto and its moon, Charon (1200 kilometers in diameter; on the right in the image above), are members of the Kuiper Belt. (Pluto is now known to have at least two other smaller moons orbiting it.)

animated gif of Pluto from HST
animation of Pluto as seen by the Hubble Space Telescope using images released Feb. 6, 2010

In July 2005 the discovery of a Kuiper Belt object larger than Pluto was announced, called Eris (formerly UB 313). Is Eris the tenth planet? If Pluto is a planet, should not Eris be considered a planet too? How about Ceres in the asteroid belt? Although the discovery of a Kuiper Belt object the size of Pluto or larger was considered likely, Eris’ discovery finally forced astronomers to decide what is to be called a “planet,” what is a “minor planet,” what is an “asteroid,” “large comet,” etc. (Note that recent measurements of Eris say that it is about the same diameter as Pluto but 20% larger in mass. It doesn’t change the “planet” definition problem.)

On August 23, 2006, the International Astronomical Union (IAU, the official authority responsible for naming stars, planets, celestial bodies and phenomena, etc.—the official body of astronomy) re-classified Pluto as a “dwarf planet.” A “planet” in our solar system is a celestial body that “(a) orbits the Sun; (b) has sufficient mass for its self-gravity to overcome rigid-body forces so that it assumes a hydrostatic equilibrium (nearly round) shape; and (c) has cleared the neighborhood around its orbit.” Pluto fits (a) and (b), but not (c). Pluto, Eris, Ceres, and others will be called “dwarf planets” because although they fit (a) and (b), they have not cleared the neighborhood around their orbits. Also, a “dwarf planet” is not a satellite (which may leave out Charon, but its large mass compared to Pluto may make Charon to be a “dwarf planet”).

The third criteria (c) of a planet from the IAU has caused a considerable amount of debate—what does “cleared the neighborhood around its orbit” mean? One interpretation is to say that the object gravitationally dominates its orbital zone where an orbital zone includes all objects whose orbits cross each other, their orbital periods differ by less than a factor of 10, and they are not in a stable resonance. Within that orbital zone, if a round object is much more massive (say, by at least 100 times) than the other objects combined mass, it will gravitationally dominate its zone. With this interpretation there is a clear separation between the eight “planets” (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune) and the “dwarf planets”. All eight planets are at least 5000 times more massive than the other objects in their orbital zones while Pluto is only 0.07 times the mass of the rest of the objects in its orbital zone (Ceres is just 0.33 times the mass of the rest in its orbital zone). At the time of writing, Haumea and Makemake were the only others that had been officially recognized by the IAU as “dwarf planets”. However, any body made mostly of ices larger than about 400 kilometers will be round, so the number of dwarf planets is undoubtedly much larger. By the time all of the bodies in the Kuiper Belt are found, the number of dwarf planets will probably number well over 200. See Mike Brown’s “The Dwarf Planets” page or his blog for more about dwarf planets and their number (he is the discoverer of Eris).

Even smaller objects (comets, most asteroids, etc.) will be called “Small Solar System Bodies”. This does leave open the question of how this applies to planets outside the solar system, especially the truly planet-sized objects that are not bound to any star. Another controversial issue behind the IAU 2006 decision was the small proportion of members who voted on the decision. After the initial series of arguments following the 2006 decision, over time the astronomers came to accept the decision and the planet definition issue did not even come up at the IAU 2009 meeting.

The image below compares Pluto, Charon to Earth and the Moon. Eris is probably the same diameter as Pluto, so it is still smaller than the Moon. In mid-January 2006, the New Horizons spacecraft was launched on a 9.5 year trek to Pluto-Charon. After flying by Pluto-Charon in July 2015, it will be directed to another Kuiper Belt object.

Earth, Moon, Pluto and Charon to the same scale (using images from NASA and USGS)

The current list of objects of the Kuiper Belt is at the Minor Planets Center (the following links will display in another window). They keep a list of the trans-Neptunian objects and a list of the Centaurs. A plot of the positions of the observed Kuiper Belt objects is also available from the Minor Planets Center.

Regardless of where it is in the solar system, the Sun’s gravity is always pulling on the comets. When a comet is close to the Sun, it moves quickly because of the great force of gravity it feels from the Sun. It has enough angular momentum to avoid crashing into the Sun. Angular momentum is a measure of the amount of spin or orbital motion an object or system of objects has—see appendix A for more on angular momentum. As a comet moves away from the Sun, the Sun’s gravity continually slows it down. Eventually, the comet slows down to the aphelion point and the Sun’s gravity pulls it back.

A comet’s motion around the Sun is sort of like a swing on the Earth. When the swing is closest to the ground, it moves quickly. As the swing moves up, the Earth’s gravity is continually pulling on it, slowing it down. Eventually, the swing is slowed down so much that it stops and the Earth’s gravity pulls it back down. The swing has enough angular momentum to avoid crashing to the ground.

10.8 Comet Beginnings and Ends

Comets formed 4.6 billion years ago along with the rest of the planets from the same solar nebula material. They were too small and cold to undergo any geologic activity (they did not differentiate), so they preserve the record of the early solar nebula composition and physical conditions. Those forming near the Jovian planets were deflected outward, swallowed up or sent careening inward toward the terrestrial planets and the Sun. The water originally on the forming terrestrial planets may have evaporated into space, so the water now present on the terrestrial planets may have come from comets crashing into them.

Short period comets make hundreds to thousands of passes around the Sun spewing out gas and dust. Over time a comet will leave bits of dust along its orbit, each piece of dust has an orbit close to the comet’s orbit. The dust grains are the size of a grain of sand or smaller. If the Earth passes through the comet’s orbit, the dust grains can hit the Earth’s atmosphere to make the spectacular displays called meteor showers. After many passages around the Sun, the nucleus has no more volatile material and it becomes “dead.”

comet bits make meteor showers

The famous Perseid meteor shower in mid-August is due to Earth passing through the orbit of Comet Swift-Tuttle and the Leonid meteor shower in mid-November is due to Comet Tempel-Tuttle. The meteor showers appear to be coming from a particular direction in the sky so the meteor showers are named after the constellation they appear to be coming from. The Perseids appear to diverge from the Perseus constellation and the Leonids diverge from Leo. When the parent comet passes through the inner solar system, the meteor shower display is particularly impressive—several hundred meteors can be seen in one hour. Such events are called meteor storms. The last storm was for the Leonids in 1966. Another Leonid storm was predicted in either 1997, 1998 or 1999. Although several hundred meteors were seen per hour during 1998 and 1999, the shower was not the spectacular storm hoped for. While most comet dust particles in meteor showers hit the atmosphere at 30 to 40 kilometers/second, the Leonid particles hit our atmosphere at 72 kilometers/second. The table summarizes the characteristics for several impressive showers. A meteor shower will cover a time period of several days before and after the given maximum date.

Some major meteor showers
Max. Date Meteors
per hour
RA :: Dec
Quadrantids Jan. 3 40 to 110 Bootes, Hercules, Draco
15h 28m :: +50°
2003 EH1
Lyrids Apr. 22 8 to 12 Hercules-Lyra
18h 16m :: +34°
C/1861 G1
Eta Aquarids May 5 5 to 20 Aquarius
22h 24m :: 0°
Comet Halley
Delta Aquarids Jul. 28 15 to 35 Aquarius
22h 36m :: –17°
Perseids Aug. 11 to 12 40 to 70 Perseus
03h 04m :: +58°
Comet Swift-Tuttle
Orionids Oct. 21 13 to 30 Orion
06h 20m :: +15°
Comet Halley
Taurids Nov. 1 to 8 5 to 12 Taurus
03h 32m :: +14°
Comet Encke
Leonids Nov. 17 6 to 10 Leo
10h 08m :: +22°
Comet Temple-Tuttle
Geminids Dec. 13 to 14 50 to 70 Gemini
07h 32m :: +32°
3200 Phaethon
(inactive comet?)

More on meteor showers from:

The meteors not associated with a meteor shower are bits of rock from asteroids. The meteors that ARE associated with a meteor shower are much too fragile to survive their trip through our atmosphere and burn up at altitudes above 50 kilometers. Some of the comet dust intercepts the Earth at much slower speeds than those making the meteors and can make its way to the surface gently. Don Brownlee, an astronomer at the University of Washington has pioneered the collection of this comet dust in the stratosphere. More information about the comet dust samples is available at the Stratospheric Dust web site at the Planetary Materials Curation office of NASA.

10.8.1 Other Comet Sites on the Web

The following links will display in another window with most browsers.

a. Solar System Tours departure points page. Links to comet, asteroid, and meteorite tours are given toward the bottom of the table.
b. NASA/JPL’s comet and Kuiper Belt sections of their Solar System Exploration site.
c. Sky and Telescope‘s Comet observing guide. Includes starcharts and pictures of visible comets.

Sections Review


  • angular momentum
  • dust tail
  • ion tail
  • Kuiper Belt
  • long period comet
  • meteor shower
  • nucleus (comet)
  • Oort Cloud
  • primitive
  • short period comet
  • solar wind
  • sublime

Review Questions 2

  1. What is a comet?
  2. How do comets give clues to the original conditions of the solar system?
  3. If all of the objects in our solar system (Sun, planets, moons, etc.) formed from the same material, why are most meteorites and comets useful for finding out what the early solar system was like but the planets and the Sun are not?
  4. What are the four components of a comet when it is close to the Sun and what are their dimensions?
  5. Put the nucleus of a typical comet in the following sequence: stadium, Bakersfield, California, United States, Earth. Why can the nucleus not hang onto its gas and dust?
  6. What unexpected things did we find out about the nuclei of comets from recent comet missions? Why were they unexpected?
  7. What are comets made of and what is the structure of the nucleus like?
  8. What happens to a comet’s nucleus as it approaches the Sun?
  9. What are the two tails of a comet and what are they made of? What gives them their characteristic colors?
  10. Which way do the tails point? Why is a comet’s tail in front of a comet as it moves away from the Sun?
  11. How are long period comets associated with the Oort Cloud? How is the Oort Cloud known to exist if it has not been observed?
  12. What direction can long period comets come from? What causes a comet in the Oort cloud to head toward the inner solar system?
  13. How are short period comets associated with the Kuiper Belt?
  14. What direction do most short period comets come from?
  15. How were the Oort Cloud and Kuiper Belt formed?
  16. How are the meteors in a meteor shower different from the ordinary meteors you can see on any night of the year?
  17. Why does a meteor shower happen at the same time every year?

10.9 Solar System Formation

The radioactive dating of meteorites says that the Sun, planets, moons, and solar system fluff formed about 4.6 billion years ago. What was it like then? How did the solar system form? There are some observed characteristics that any model of the solar system formation must explain.

10.9.1 Observables

a. All the planets’ orbits lie roughly in the same plane.
b. The Sun’s rotational equator lies nearly in this plane.
c. Planetary orbits are slightly elliptical, very nearly circular.
d. The planets revolve in a west-to-east direction. The Sun rotates in the same west-to-east direction.
e. The planets differ in composition. Their composition varies roughly with distance from the Sun: dense, metal-rich planets are in the inner part and giant, hydrogen-rich planets are in the outer part.
f. Meteorites differ in chemical and geologic properties from the planets and the Moon.
g. The Sun and most of the planets rotate in the same west-to-east direction. Their obliquity (the tilt of their rotation axes with respect to their orbits) are small. Uranus and Venus are exceptions.
h. The rotation rates of the planets and asteroids are similar—5 to 15 hours, unless tides slow them down.
i. The planet distances from the Sun obey Bode’s law—a descriptive law that has no theoretical justification. However, Neptune is a significant exception to Bode’s “law.”
j. Planet-satellite systems resemble the solar system.
k. The Oort Cloud and Kuiper Belt of comets.
l. The planets contain about 90% of the solar system’s angular momentum but the Sun contains over 99% of the solar system’s mass.

10.9.2 Condensation Model

The model that best explains the observed characteristics of the present-day solar system is called the Condensation Model. The solar system formed from a large gas nebula that had some dust grains in it. The nebula collapsed under its own gravity to form the Sun and planets. What triggered the initial collapse is not known. Two of the best candidates are a shock wave from a nearby supernova or from the passage through a spiral arm. The gas cloud that made our solar system was probably part of a large star formation cloud complex. The stars that formed in the vicinity of the Sun have long since scattered to other parts of the galactic disk. Other stars and planets in our galaxy form in the same basic way as will be described here. The figure below summarizes the basic features of the Condensation Model. After the figure is further explanation of the model and how it explains the observable items in the previous section.

basic features of the solar system formation model

A. A piece of a large cloud complex started to collapse about five billion years ago. The cloud complex had already been “polluted” with dust grains from previous generations of stars, so it was possible to form the rocky terrestrial planets. As the piece, called the solar nebula collapsed, its slight rotation increased. This is because of the conservation of angular momentum.
B. Centrifugal effects caused the outer parts of the nebula to flatten into a disk, while the core of the solar nebula formed the Sun. The planets formed from material in the disk and the Sun was at the center of the disk. This explains items (a) and (b) of the observables above. Disks are seen around most of the young lower-mass stars in our Galaxy today, so we know that this part of the model is a common process in star formation.
C. Most of the gas molecules and dust grains moved in circular orbits. Those on noncircular orbits collided with other particles, so eventually the noncircular motions were dampened out. The large scale motion in the disk material was parallel, circular orbits. This explains items (c) and (d) of the observables above.
D. As the solar nebula collapsed, the gas and dust heated up through collisions among the particles. The solar nebula heated up to around 3000 K so everything was in a gaseous form. The solar nebula’s composition was similar to the present-day Sun’s composition: about 93% hydrogen, 6% helium, and about 1% silicates and iron, and the density of the gas and dust increased toward the core where the proto-Sun was. The inner, denser regions collapsed more quickly than the outer regions.
When the solar nebula stopped collapsing it began cooling, though the core forming the Sun remained hot. This meant that the outer parts of the solar nebula cooled off more than the inner parts closer to the hot proto-Sun. Only metal and rock materials could condense (solidify) at the high temperatures close to the proto-Sun. Therefore, the metal and rock materials could condense in all the places where the planets were forming. Volatile materials (like water, methane and ammonia) could only condense in the outer parts of the solar nebula. This explains item (e) of the observables above.
Around Jupiter’s distance from the proto-Sun the temperature was cool enough to freeze water (the so-called “snow line” or “frost line” ). Further out from the proto-Sun, ammonia and methane were able to condense. There was a significant amount of water in the solar nebula. Because the density of the solar nebula material increased inward, there was more water at Jupiter’s distance than at the distances of Saturn, Uranus, or Neptune. The greater amount of water ice at Jupiter’s distance from the proto-Sun helped it grow larger than the other planets. Although, there was more water closer to the Sun than Jupiter, that water was too warm to condense.
Material with the highest freezing temperatures condensed to form the chondrules that were then incorporated in lower freezing temperature material. Any material that later became part of a planet underwent further heating and processing when the planet differentiated so the heavy metals sunk to the planet’s core and lighter metals floated up to nearer the surface. Observables item (f) is explained.

terrestrial vs jovian condensation regions

E. Small eddies formed in the disk material, but since the gas and dust particles moved in almost parallel, near-circular orbits, they collided at low velocities. Instead of bouncing off each other or smashing each other, they were able to stick together through electrostatic forces to form planetesimals. The larger planetesimals were able to attract other planetesimals through gravity and increase in size. This process is called accretion.
The coalescing particles tended to form bodies rotating in the same direction as the disk revolved. The forming planet eddies had similar rotation rates. This explains items (g) and (h) above. The gravity of the planetesimals tended to divide the solar nebula into ring-shaped zones. This process explains item (i) above.
F. More massive planetesimals had stronger gravity and could pull in more of the surrounding solar nebula material. Some planetesimals formed mini-solar nebulae around them which would later form the moons. This explains item (j) above. The Jupiter and Saturn planetesimals had a lot of water ice mass, so they swept up a lot of hydrogen and helium. The Uranus and Neptune planetesimals were smaller so they swept up less hydrogen and helium (there was also less to sweep up so far out). The inner planetesimals were too small to attract the abundant hydrogen and helium.
G. The small icy planetesimals near the forming Jupiter and Saturn were flung out of the solar system. Those near Uranus and Neptune were flung to very large orbits. This explains the Oort Cloud of item (k) above. There was not enough material to form a large planet beyond Neptune. Also, accretion of material at these great distances progressed more slowly than material closer to the Sun. The icy planetesimals beyond Neptune formed the Kuiper Belt. The large planets were able to stir things up enough to send some of the icy material near them careening toward the terrestrial planets. The icy bodies gave water to the terrestrial planets.
H. The planets got big enough to retain heat and have liquid interiors. The heavier materials like iron and nickel sank to the planet cores while the lighter materials like silicates and gases rose toward the surface, in a process called differentiation. The sinking of the heavy material created more heat energy. The planets also had sufficient radioactive decays occurring in them to melt rocky material and keep it liquid in the interior. The small planetesimals that were not incorporated into the large planets did not undergo differentiation. This explains item (f) of the observables.
I. The proto-Sun had a magnetic field and spewed out ions. The ions were dragged along by the magnetic field that rotated with the proto-Sun. The dragging of the ions around slowed down the proto-Sun’s rotation rate. Also, accretion disks like the solar nebula tend to transfer angular momentum outward as they transfer mass inward. This explains item (l) above.
J. Because of its great compression, the core of the proto-Sun core reached about 10 million Kelvin and the hydrogen nuclei started fusing together to produce helium nuclei and a lot of energy. The Sun “turned on.” The Sun produced strong winds called T-Tauri winds that swept out the rest of the nebula that was not already incorporated into the planets. This whole process took just a few hundred million years and was finished by about 4.6 billion years ago.

After forming the disk, the disk would have cooled as the heat was radiated to space. However, before ices could condense and clump closer to the proto-Sun than the “frost line,” the Sun went through the T-Tauri stage and the strong winds swept out the remaining gas including the hydrogen compounds. If the nebula had cooled more quickly, the inner planets might have been able to get bigger and have more hydrogen compounds in them. If the nebula was warmer, then the “frost line” would have been further out, so Jupiter would have been a terrestrial planet. The ice cores forming beyond about Neptune’s distance never got big enough to capture the surrounding hydrogen and helium gas—they stayed small to become the dwarf planets such as Pluto, Eris, Makemake, Haumea, etc. and the other Kuiper Belt Objects.

Although the Condensation Model explains a number of observed facts of the properties of the present-day solar system, we have been looking at a sample size of just one: our solar system. It has been only relatively recently that we have been able to test the theory with other forming and fully-developed planetary systems with the placement of powerful telescopes above the Earth’s atmosphere. Observations in the infrared and sub-millimeter wavelengths have enabled us to peer through the thick dust that shields forming stars and their planetary disks. In some cases some of the dust/gas cocoons have been stripped away by ultraviolet from nearby very hot stars so we can study them in optical wavelengths. These star formation processes will be explored a bit more fully in another chapter, but I will note briefly here that the processes described above are seen in other forming planetary systems and a great majority of forming stars have flattened disks with them so planet formation (or at least the “solar nebula” disk formation part) is a common process. We have used those observations to modify and improve the Condensation Model. In particular our observations of other planetary systems has forced us to seriously explore how planets can migrate inward from where they first started forming. Perhaps, one should not be surprised that that could happen. As described in item G above, gravitational interactions flung chunks of material all about. On the other hand, those chunks were much smaller than the forming planets. As described in the next section, the observations show that gravitational interactions could also shift things as big as the planets themselves.

Review Questions 3

  1. Why is the Condensation Model the preferred model of the formation of the solar system?
  2. From what did the solar system form?
  3. Why are the inner terrestrial planets small and rocky while the outer jovian planets are large and gaseous?
  4. Why does a disk form in the collapsing cloud?
  5. What role do dust particles play in planet formation?
  6. If the disk was moving so quickly, how did it create big enough clumps to make planets?
  7. What drove out the rest of the nebula after the planets formed?
  8. Why are the planet interiors made of layers of increasing density closer to their cores?
  9. How do we test the Condensation Model of the solar system?

10.10 Testing the Theory: Other Planetary Systems

Some 455 planets have been found orbiting other stars—exoplanets (sometimes also called extrasolar planets)—in 388 exoplanet systems at the time of the last update of this webpage. This section will first look at how we find exoplanets and then I will draw some preliminary conclusions based on the statistics of the orbits and masses of the exoplanets.

10.10.1 Finding Exoplanets

Detecting planets around other stars (exoplanets) is a difficult project requiring very careful observations. At first finding planets might seem a simple thing to do—take pictures of stars and look for small faint things orbiting them. A planet would indeed be a faint: a billion or more times fainter than a star in the visible band—the glare of the starlight would wash out the feeble light of a planet. The direct imaging technique of finding planets would be better accomplished in the infrared band because the planet’s thermal spectrum would have maximum emission in the infrared band. Also, stars produce less infrared energy than visible band energy—a planet would only be ten to a hundred thousand times fainter than the star. The planet would still be very faint, but at least the contrast ratio is improved by many thousands of times. The direct imaging technique is able to find jovian planets far from their parent stars. Twenty-four planets (as of early-June 2011) have been found this way.

less contrast in the infrared

Some of the planets imaged are very young and still quite warm from their formation. Therefore, the young planets are quite bright in the infrared and easier to detect. Some planets have been imaged by blocking the light from the much brighter star with a device called a coronograph so that the feeble light from the planet can be detected. Use of a coronograph was essential to create the first visible light (optical) image of a planet: that orbiting the very bright star, Fomalhaut, shown below. The black area in the center is the coronograph, the white dot shows the location of the star, the ring is a dusty debris disk analogous to our solar system’s Kuiper Belt (but much further out), the small white box shows the location of the planet some 115 AU from its star, and the inset shows its motion over two years of its entire 872-year orbit. Its motion proved it was an object orbiting the star.

Fomalhaut B discovery image

Astronomers have detected disks of dust and gas around young stars using sensitive infrared detectors on the largest telescopes in the world. An equivalent amount of material locked up into a single object will have a smaller total surface area than if it was broken up into many tiny particles. The disks have a lot of surface area and, therefore, can emit a lot of infrared energy. Some bright stars in our sky have dust around them: Vega, Beta Pictoris, and Fomalhaut. These are systems possibly in the beginning stages of forming planets. One disk around the star HR 4796A appears to be in between the dust disk stage and a fully-fledged planet system. The inner part of the disk has been cleared away. Presumably, the dust material has now coalesced into larger things like planets. The planets would have a smaller surface area than if the material was still in tiny particles form, so the planets will be much fainter. The Hubble Space Telescope has also detected disks of gas and dust around 50% of the stars still forming in the Orion Nebula. It appears that the formation of planet systems is a common process in the universe.

Another way to look for exoplanets is to notice their gravitational effect on the stars they orbit. One signature of a planet would be that the star would appear to wobble about as the star and the planet orbit a point situated between them, proportionally closer to the more massive star, called the center of mass. This technique is called the astrometric technique. Our Sun wobbles because of the gravity of the planets orbiting it. Most of the wobble is due to Jupiter which contains more mass than all of the other planets combined. However, the wobble is tiny! Because the Sun is over a thousand times more massive than Jupiter, the center of mass is over a thousand times closer to the Sun, or about 47,000 kilometers above the surface of the Sun (this distance is less than 7% of the radius of the Sun). Despite the tiny wobble, astronomers on planets orbiting nearby stars could detect this wobble using the same technology we have here on Earth if they observed the Sun’s motion very carefully over a couple of decades. The stronger the gravity between the star and planet, the larger will be the wobble of the star and the easier to detect. Therefore, the astrometric technique is well-suited to finding massive Jovian exoplanets close to their parent stars. No exoplanets have been found using this technique (at the time of writing). The now-canceled SIM Lite mission was to use this technique and the Gaia mission, scheduled to launch in 2013, will use this technique.

finding planets from the slight motions of the stars they orbit
Sequence on the right side is actually from two different vantage points. The wobbling star is what you would see if the orbit was face-on. The Doppler shifting absorption lines is what you would see if the orbit was edge-on from a position to the right of the star-planet system (so the lines shift toward the red end when the star is moving away from the observer and the exoplanet is moving toward the observer).

Another signature of an exoplanet would be Doppler shifts in the star’s spectral lines as they orbit their common center of mass. The Doppler shift technique (also sometimes called the radial velocity technique) has been the easiest and most prolific way to find exoplanets so far. As of the time of writing over 500 exoplanets have been found using the Doppler shift technique. The searches have so far focused on stars similar to the Sun, though two systems have planets orbiting a pulsar (a type of ultra-compact, dead star discussed in the stellar development chapter—planets found using a variation of the Doppler shift technique called the timing technique), ten systems have M-type red dwarf stars (including one that has a terrestrial-sized planet in its habitable zone), four systems have brown dwarfs, four systems have A-type stars, and three have B-type stars. Like the astrometric technique, the Doppler shift technique is well-suited to finding massive Jovian planets close to their parent stars. The number of systems discovered and the details about them changes so rapidly that the best place to find up-to-date information on exoplanets is on the internet. Some websites are given at the end of this chapter.

The period of the star wobble is measured and then the distance (semi-major axis of the orbit) is derived from Kepler’s third law. The star’s velocity change is measured and then the total mass of the system is derived from Newton’s Laws of Motion. We can estimate the mass of the star from its spectral type, estimate the planet velocity from the star wobble period and then derive the exoplanet’s mass. However, the Doppler effect tells you about the motion along the line of sight only. The exoplanet orbits are undoubtedly inclined, or tipped, to our line of sight and the amount of inclination is uncertain. This introduces an uncertainty in the derived masses of the exoplanets. Usually, astronomers will quote the masses as “mass × sin(orbit inclination angle),” so the actual exoplanet masses could be higher. The star-wobble techniques can also give us the orbit eccentricity if we have observations from the entire orbit.

Astronomers cannot yet determine the diameters of most of the giant exoplanets, so their densities, and, therefore, their compositions, are still unknown. Eighty-one of the giant planets have been observed to move in front of their stars and cause an eclipse or dimming of the starlight. This is called a transit so this means of detecting planets is called the transit technique. A transit means that the planet’s orbit is aligned with our line of sight (and the inclination angle is nearly 90 degrees). From the planet transit, astronomers have been able to accurately measure the diameter of the planet. Using the planet mass from the star wobble methods you can then determine the density. Careful observations of the spectrum of the star while the planet is transiting across will enable astronomers to determine the chemical composition of the planet’s atmosphere using spectroscopy. In other cases, the planet’s spectrum is found from taking the spectrum of the star+planet, then taking the spectrum of just the star when the planet is behind the star and subtracting it from the star+planet spectrum. One planet, HD 189733b, has water, methane and carbon dioxide in its atmosphere, but the planet is much too hot and massive to support life. It was not until January 2010 that astronomers had been able to take the spectrum of a planet directly—an important step in eventually being able to analyze the spectrum of a terrestrial planet to see if it is supporting life on it.

big planet transitting makes bigger dip in brightness than smaller planet

Most of the transiting planets were first detected via the Doppler shift technique, but the transit technique can be another way of searching for planets around other stars. However, most planetary systems do not have their orbits so exquisitely aligned with our line of sight, so a lot of stars would need to be looked at to improve the chances of finding even a few transits. One advantage of the transit technique over the star-wobble methods for planet detection is that you would be able to detect terrestrial-diameter planets (i.e., small planets). Small planets like the Earth produce too small a wobble in their parent star (because of their small mass) to be detected by the star-wobble methods. The COROT mission (ESA) has found a planet less than twice the diameter of the Earth. However, this planet is so close to its star that the planet’s surface temperature is 1000 to 1500 deg C! The NASA/JPL spacecraft mission called Kepler is looking at about 156,000 stars simultaneously to search for Earth-sized planets during a 3.5-year period of time. The spacecraft is focusing on planets that could be in the stars’ habitable zones (where liquid water could exist on a planet surface). Only 0.5% of the stars are expected to have their planets’ orbits in the habitable zones properly aligned for detection by Kepler. A terrestrial planet with mass between 0.5 to 10 Earth masses will cause its star to dim by a fractional amount of between 0.00005 to 0.0004, respectively, and the transits will last just a few hours. The 3.5-year time period was determined from the need to verify the repeatability of the planet transiting at least two more times after the first detected transit with the same time interval between transits and depth of transit. Such repeatability of the transits would mean that something was orbiting the star and not just some chance occurrence of an unrelated object passing in front of the star. For a solar-type star with a planet in the habitable zone, the planet would transit the star once a year. However, this assumes that the stars are calm and steady like our Sun. The Kepler team has found that a number of the stars are a bit more active, more variable, than our Sun, so they will need more observations to tease out the dimmings due to transiting planets from those due to the intrinsic variability of the stars themselves. Will NASA have the funds to continue the mission beyond the 3.5-year original mission?

The Kepler team has created some nice interactives showing how the planet detection works as well as how the various planet parameters are derived. As of early 2011, Kepler had found over 1200 planetary candidates with almost 410 of them in 170 planetary systems with multiple planets. Candidate planets are those that have not been verified yet through follow-up observations to make sure the star dimming is not due to another star, as in an eclipsing binary system or a dead star called a white dwarf. Sixteen planets have been confirmed, including one that is definitely a rocky planet with a density of 8.8 times that of water called Kepler 10b. However, Kepler 10b orbits less than 0.017 AU from its star (Mercury orbits our Sun at 0.39 AU), so its surface temperature is over 1800 K! Of the candidates (as of early 2011), 68 have diameters less than 1.25 Earth’s, and 54 candidates reside in their star’s habitable zone and have diameters ranging from Earth-size to larger than Jupiter with one having the sought-for combination of Earth size in the star’s habitable zone (five in the habitable zone have sizes from 0.9 to 2 times the diameter of the Earth). The search continues!

Another method of planet detection uses the gravitational lensing effect discussed in the Einstein’s Relativity chapter. When a star passes almost in front of another more distant star as seen from the Earth (the stars are not orbiting each other), the light from the distant star can be warped and focused toward us by the gravity of the nearer star to produce multiple images of the distant star or even a ring of light if they are aligned exactly right. This lensing effect is too small and the resolving powers of telescopes are too small to see the multiple images. The multiple images will blend together into a single blurry blob that is brighter than when the multiple images are not present (a microlens event). As the nearer star moves in front of the distant star, the nearer star’s blurry blob will appear to brighten and then dim as the nearer star moves out of alignment. The microlens event for typical stars in our galaxy moving at typical speeds will last a few weeks to a few months and the amount of the brightness magnification will depend on how closely the near and distant stars are aligned with our line of sight.

close up view of microlens event

The animation above shows an extremely-magnified view of two possible microlens events (what you would see if you had an optical telescope several hundred meters across in space). The brightness of the ring and the combined brightness of the two distorted images exceed the distant star’s brightness when it is not lensed. This animation is adapted from a figure by Penny Sackett in a talk about the search for planetary systems using microlenses.

If the nearer star has a planetary system with a planet at the right position, a smaller and briefer microlens event will happen superimposed on top of the star’s microlens. By looking for brief deviations in the otherwise smooth increase, then smooth decrease of a stellar microlens event, you could detect the presence of a planet. This method is called the microlens technique and is summarized in the figure below—select the image to view the full-size version in another window. The planet’s mass and orbit size could be determined from careful measurements of the brief deviations. The microlens event method can be used to detect jovian-mass and terrestrial-mass planets near their parent stars and the parent stars are distant from the Earth. Like the transit method, a lot of stars must be monitored to pick up even a single stellar microlens event. The microlens events are due to chance alignments that are not repeatable. Thirteen planets orbiting stars have been found using the microlens technique at the time of writing.

microlens planet technique geometry

In May 2011, two teams using the microlens technique announced the discovery of several other planets that are not orbiting a star—”free floaters.” The teams had observed about 50 million stars in the direction of the Milky Way’s bulge every 10 to 50 minutes in 2006 and 2007 looking for those chance alignments. They found a surprisingly large number of brightenings caused by planet-mass objects alone. A statistical extrapolation of the results says that the free floater planets could be almost twice as numerous as normal main sequence stars in the Galaxy.

The transit and microlens techniques are not good for looking planets around a particular star of interest. The star-wobble and direct imaging methods are better. However, the transit and microlens methods are useful for determining the statistics of planetary systems in our galaxy, particularly the number of star systems with terrestrial planets in the habitable zones. Another possible exoplanet detection method uses the amount of lithium in a star. A comparison of stars with planets and those stars without planets shows that the stars with planets have about 1% of the lithium in the star than in stars without planets. Such a detection method could offer a much more cost-effective to search for planetary systems than the other techniques being used now.

10.10.2 Results and Testing the Theory

The left figure below summarizes the orbit sizes of the 801 exoplanets with known orbit sizes as of January 2013 from the Extrasolar Planets Encyclopedia. The right figure is for the 680 other planetary systems with known eccentricities. Most of these exoplanets are Saturn-Jupiter mass or larger and most of those that transit their stars have densities like Saturn-Jupiter or less. The Kepler mission has found a few smaller exoplanets so far, including those with terrestrial planet-like densities, and within several years, their number is expected to be well over a hundred. In the orbit size plot, the large exoplanets (those with mass greater than or equal to half of Jupiter’s mass, “Mjup”) are the blue bars and the smaller exoplanets are the red bars plotted on top so that the total number of exoplanets for a given orbit size is simply the total height of the blue plus red.

orbit size histogram orbit eccentricity histogram

Two things to notice are how close the large exoplanets (50% Jupiter’s mass or larger = blue bar) are to their stars and the large eccentricities of some of the exoplanet orbits. The large exoplanets very close to their stars (within 0.5 AU) are called “hot Jupiters” because their temperatures can get up to 1000 deg C in their cloudtops (the clouds would probably be made out of rock-dust minerals instead of the ammonia, ammonium hydrosulfide, and water clouds of the much colder Jupiter and Saturn). The hot Jupiters with low densities have atmospheres puffed out by the extreme solar heating—that inflates their diameter.

The Condensation Model outlined in the previous section predicts that large planets will only form far from the young star. Giant planets start from a core of rock and ices that were able to solidify far from the intense heat of the young star. The rock-ice cores then pull in surrounding gas by their gravity. Near the star, the temperature is too high to form the rock-ice cores.

Over a decade before the discovery of the first exoplanets, astronomers predicted as part of the Condensation Model that large gas/rock clumps would form far from a young star and spiral inward toward the star because of friction with the gas remaining in the disk around the forming star. The gas/rock clumps can also interact with each other sending one into a small orbit while the other is ejected out of the system. Such interactions may also explain the elliptical orbits we see. Some astronomers working on planet formation models are looking for ways to halt the inward spiral of the gas giant planets near the star through tidal interactions between the planet and star. Perhaps the gas giant planets we see are simply the ones that did not have time to spiral completely into the stars before the gas disk was cleared away by the strong T-Tauri winds that accompany the start of nuclear fusion. Perhaps in our solar system other giant planets had formed but did not survive or were ejected. Evidence for the ejection possibility comes from the potentially large number of free floater planets that the microlens surveys are saying must exist in the Galaxy. Recent computer simulations of the dynamical history of our solar system show that the gravity of Saturn helped prevent Jupiter from spiraling into the Sun and that their orbits may have started further out than they are now, then moved closer in than they are now, and then finally moved further out to their present distances. The simulations also show that Uranus’ initial orbit might have been larger than Neptune’s initial orbit and that both planets’ orbits were smaller than they are now. This shuffling of the gas giant planets would also have affected the material forming the terrestrial planets and changed the distributions of various types of asteroids and comets. Observations of other star/planet formation places and other planetary systems along with more sophisticated computer simulations have confirmed various features of the Condensation Model and they have also led to modifications and extensions of the theory in the continual interaction of the observation – theory-testing – error correction process of science.

The Kepler mission has provided strong evidence in favor of the inward migration idea for how the hot Jupiter systems formed. A recent study looked at over 60 hot Jupiter systems in the Kepler catalog and none of them had multiple planets while other systems with large planets further out can have multiple planets. Another study investigated Kepler-30, a non-hot Jupiter system, and were able to determine that the star’s rotation is aligned with the orbits of the three planets, just like our Sun’s equator is aligned with the planets in our solar system. The hot Jupiter systems usually have orbits mis-aligned with the star’s rotation because of the gravity tugs from other former planets flung out by the hot Jupiter as it spiraled in. Kepler-30 is just one system so future work will need to be done on other systems to confirm or negate this conclusion.

One puzzling statistic from the Kepler mission has to do with the sizes (diameters) of the exoplanets. More than three-fourths of the planet candidates in the Kepler catalog have sizes ranging between that of the Earth and Neptune. Why doesn’t our solar system have a planet in that size range? In that respect, our solar system’s architecture seems to be an unusual one in the Galaxy. Further refinement of the Condensation Model will need to be made to explain why super-Earths/mini-Neptunes are so common and what happened in our solar system to prevent such a planet from forming or continue to exist in our solar system.

In the next few years, ground-based interferometers will be completed that can image large exoplanets. What about Earth-like planets? It is unlikely that life could arise on a gas giant planet. NASA’s proposed Terrestrial Planet Finder (TPF), a space-based mission, should be able to obtain infrared or optical pictures of life-bearing planets. With TPF astronomers will also be able to analyze the spectrum of the planets to determine the composition of their atmospheres. Spectral lines from water would say that a planet has a vital ingredient for life. If oxygen, particularly ozone (a molecule of three oxygen atoms), is found in the atmosphere, then it would be very likely that life is indeed on the planet. Recall from the previous chapter that molecular oxygen quickly disappears if it is not continually replenished by the photosynthesis process of plants and algae. However, it is conceivably possible for a few non-biological processes (e.g., the runaway greenhouse effect with the photodissociation of carbon dioxide and water) to create an atmosphere rich in molecular oxygen and molecular oxygen does not produce absorption lines in the preferred infrared band that would be used in the TPF mission. Ozone does. Ozone existing along with nitrous oxide and methane in particular ratios with carbon dioxide and water, all of which produce absorption lines in the infrared, would be very strong evidence for an inhabited world.

The setup and technologies TPF will employ will be based on the experience gained from previous projects such as the Keck Interferometer, the Large Binocular Telescope InterferometerKepler,CoRot, and the Gaia Mission. Unfortunately, there are now no plans to develop TPF for at least the next decade.

Exoplanets Web Sites

The number of stars with detected planets and the details about them changes so rapidly that the best place to find up-to-date information on exoplanets is on the internet. Here are some WWW links (will display in another window):

  1. An excellent starting point is the Extrasolar Planets Encyclopedia This site is maintained by Jean Schneider of Observatorie de Paris (it is in English, though).
  2. Planet Quest: the search for another Earth (NASA-JPL).
  3. The Terrestrial Planet Finder — The Terrestrial Planet Finder Book.
  4. The California Planet Survey. Their Exoplanet Orbit Database uses more stringent criteria than the Extrasolar Planets Encyclopedia for what exoplanets will be put in their database. In early January 2013, the EOD had 678 exoplanets but the overall features of the EOD bar charts look the same as the bar charts in the previous section above, so the conclusions of the previous section above are still valid.
  5. NASA Exoplanet Archive includes all objects with a mass less than or equal to 30 Jupiter masses and whose orbital and/or physical properties are available in publicly-available peer-reviewed publications. They restrict their list to those objects that are clearly detected.
  6. How to Find an Extrasolar Planet (ESA).
  7. The Habitable Exoplanets Catalog from the University of Puerto Rice at Arecibo is focussed on potential habitable exooplanets discoveries. It uses an “Earth Similarity Index” (ESI) to rank exoplanets in habitable zones. The ESI is uses a combination of factors for the index: a set of “interior terms” that includes the mean radius and bulk density and a set of “surface terms” that includes the escape velocity and surface temperature with a greater weight given to the surface terms. Word of caution with the catalog is that includes some exoplanets who detections are controversial (e.g., Gliese 581g) or unconfirmed (e.g., Tau Ceti) as of early January 2013.

Section Review


  • astrometric technique
  • center of mass
  • direct imaging technique
  • Doppler shift technique
  • exoplanet
  • microlens technique
  • transit technique

Review Questions 4

  1. What are two signatures of a planet in the starlight?
  2. Why is it better to search for planets in the infrared, rather than the optical band?
  3. What sort of planets are the star wobble methods best suited to find? Why?
  4. What planet properties and orbit properties can you find with the star wobble methods?
  5. What planet detection methods could detect Earth-mass or Earth-size planets? Why would the other methods not be able to find small planets like the Earth?
  6. What planet detection methods can give us the diameter, density, and maybe composition of an exoplanet?
  7. If you wanted to search for planets around a particular star, which method(s) should you use? Why is that?
  8. What challenges to the standard condensation model do the other planetary systems give? What is a likely explanation?
  9. What would be a good way to search for Earth-like planets around other stars? How could you tell if life was probably present on an extrasolar planet?

10.11 More Solar System References

Introductory Planets Course

The University of Washington Astronomy department has an excellent web page for their introductory planets course, Astronomy 150. If you need more information about the solar system than what I have in my notes, then that is the place to check next.

Tours of the Planets

Starting points for excellent tours of each of the planets and their moons and the solar system “fluff” is given on the Planet Links page.

Is this page a copy of Strobel’s Astronomy Notes?